R.J. Loewen

A multi-moded rf delay line distribution system for the next linear collider

The Delay Line Distribution System (DLDS) is an alternative to conventional pulse compression, which enhances the peak power of rf sources while matching the long pulse of those sources to the shorter filling time of accelerator structures. We present an implementation of this scheme that combines pairs of parallel delay lines of the system into single lines. The power of several sources is combined into a single waveguide delay line using a multi-mode launcher. The output mode of the launcher is determined by the phase coding of the input signals. The combined power is extracted from the delay line using mode-selective extractors, each of which extracts a single mode. Hence, the phase coding of the sources controls the output port of the combined power. The power is then fed to the local accelerator structures. We present a detailed design of such a system, including several implementation methods for the launchers, extractors, and ancillary high power rf components. The system is designed so that it can handle the 600 MW peak power required by the NLC design while maintaining high efficiency.

SLAC/CERN high gradient tests of an X-band accelerating section

High frequency linear collider schemes envisage the use of rather high accelerating gradients: 50 to 100 MV/m for X-band and 80 MV/m for CLIC. Because these gradients are well above those commonly used in accelerators, high gradient studies of high frequency structures have been initiated and test facilities have been constructed at KEK, SLAC and CERN. The studies seek to demonstrate that the above mentioned gradients are both achievable and practical. There is no well-defined criterion for the maximum acceptable level of dark current but it must be low enough not to generate unacceptable transverse wakefields, disturb beam position monitor readings or cause RF power losses. Because there are of the order of 10,000 accelerating sections in a high frequency linear collider, the conditioning process should not be too long or difficult. The test facilities have been instrumented to allow investigation of field emission and RF breakdown mechanisms. With an understanding of these effects, the high gradient performance of accelerating sections may be improved through modifications in geometry, fabrication methods and surface finish. These high gradient test facilities also allow the ultimate performance of high frequency/short pulse length accelerating structures to be probed. This report describes the high gradient test at SLAC of an X-band accelerating section built at CERN using technology developed for CLIC.

A semi-automated system for the characterization of NLC accelerating structures

A system for characterizing the phase shift per cell of a long X-band accelerator structure is described. The fields within the structure are perturbed by a small cylindrical metal bead pulled along the axis. A computer controls the bead position and processes the data from a network analyzer connected to the accelerator section. Measurements made on prototype accelerator sections are described, and they are shown to be in good agreement with theory.

The Next Linear Collider test accelerator's RF pulse compression and transmission systems

The overmoded RF transmission and pulsed power compression system for SLACs Next Linear Collider (NLC) program requires a high degree of transmission efficiency and mode purity to be economically feasible. To this end, a number of new, high power components and systems have been developed at X-band, which transmit RF power in the low loss, circular TE01 mode with negligible mode conversion. In addition, a highly efficient SLED-II pulse compressor has been developed and successfully tested at high power. The system produced a 200 MW, 250 ns wide pulse with a near-perfect flat-top. In this paper we describe the design and test results of the high power pulse compression system using SLED-II.

Status of X-band standing wave structure studies at SLAC

Accelerating gradient is one of the major parameters of a linear accelerator. It determines the length of the accelerator and its power consumption. The SLAC two-mile linear accelerator uses 3 meter long S-band traveling wave (TW) accelerating structures. The average gradient in the linac is about 20 MV/m. This gradient corresponds to a maximum surface electric field of about 40 MV/m. An operational gradient of 40 MV/m was reported for a 1.5 m constant impedance TW structure for the SLC positron injector. This corresponds to a maximum surface field of 80 MV/m. A typical operational gradient for standing wave (SW) structures of a medical linear accelerator is 30 MV/m, with surface electric fields of 130 MV/m at a pulse width of several microseconds (longer than the working pulse width for SLAC TW structures). SW structures for S-band rf guns routinely operate at maximum surface fields of 130 MV/m (/spl sim/2 /spl mu/s pulse width). We emphasize an operational gradient with a very low fault rate in comparison to much higher gradients obtained in dedicated high gradient test structures. The operational surface fields in the above mentioned SW structures are obviously higher than in TW, S-band structures. Design considerations, results of high power tests and future plans are discussed in this paper.

RDDS cell design and optimization for the linear collider linacs

Each of the JLC/NLC main linacs will consist of /spl sim/1 million complex 3D accelerating cells that make up the 1.8 meter Rounded Damped Detuned Structures (RDDS) along its eight kilometer length. The RDDS is designed to provide maximum accelerating gradient to the beam while being able to suppress the long-range transverse wakefields to a satisfactory level. Using the 2D finite element code, Omega2, a 15% improvement in shunt impedance is found by changing the basic cell shape from a straight cylinder to a round outer wall contour that connects to slightly bulging circular disk noses. The HOM damping manifold is then designed around this optimal cell shape to improve the cell-to-manifold coupling for the dipole mode and the vacuum conductance under the frequency and minimal Q-reduction constraints for the fundamental mode. We use both MAFIA and the 3D finite element Omega3 code for this step to obtain a manifold geometry that consists of a round waveguide with additional narrow coupling slots that cut into the cell disks. As a time and cost saving measure for the JLC/NLC, the RDDS cell dimensions are being determined through computer modeling to within fabrication precision so that no tuning may be needed once the structures are assembled. At the X-band operating frequency, this corresponds to an error of a few microns in the cell radius. Such a level of resolution requires highly accurate field solvers and vast amount of computer resources. We will present calculations with the parallel code Omega3P that utilizes massively parallel computers such as the Cray T3E at NERSC. The numerical results will be compared with cold test measurements performed on RDDS prototypes that are diamond-turned with dimensions based on Omega3P simulations.